Influence of CO2 adsorption on the strength and elastic modulus of low rank Australian coal under confining pressure

• Published in: International journal of coal geology 2016-10, Vol.167, p.148-156
• Main Authors: Ranathunga, A.S., Perera, M.S.A., Ranjith, P.G.
• Format: Article
• Language: English
• Subjects: CO2 adsorption; CO2 phase and pressure; Coal rank; Strength and elastic modulus; Stress environment
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2016.08.027

Precise knowledge of changes in the CO2 adsorption-induced mechanical properties of deep coal seams is necessary for the safe and successful implementation of carbon dioxide-enhanced coal bed methane (CO2-ECBM) recovery. To date, little attention has been paid to the geo-mechanical property alterations in coal seams during CO2 adsorption under in-situ stress conditions. The aim of this study is therefore to discover how coal seam integrity varies with the introduction of CO2 under in-situ stress conditions, by conducting a series of tri-axial strength tests of Australian brown coal samples. The effect of CO2 exposure time on coal's mechanical properties was also investigated under super-critical CO2 saturation (10MPa) conditions, in order to determine the effect of long-term CO2 injection on coal seams' mechanical properties, as CO2-ECBM is a long-term process. According to the test results, irrespective of coal rank, the strength reduction in coal with CO2 injection under field conditions is significantly less than would be expected based on simple laboratory testing such as uniaxial tests. However, in any stress environment, high rank coals are subjected to greater strength and stiffness reductions with CO2 adsorption than low rank coals, due to their well-developed cleat system, and the injection of super-critical CO2 induces greater mechanical property alterations in coal than sub-critical CO2 injection. These strength and elastic modulus changes in coal with CO2 adsorption can be presented using a simple Langmuir-type equation, regardless of rank. Furthermore, the observation of the effect of long-term CO2 saturation on coal's mechanical properties revealed that, although CO2 adsorption-induced mechanical property alterations in coal are mostly completed with the first interaction with CO2, further structural re-arrangement may occur at a slower rate over time. •Coal seam integrity variation with CO2 injection under in-situ stress conditions•The effect of CO2 exposure time on coal's mechanical properties•Strength reduction in coal upon CO2 injection under field conditions is much lesser than expected.•High rank coals are subjected to greater strength reductions upon CO2 adsorption.•Such mechanical property alterations may continue for long period after the injection.